1. Field
The present invention relates generally to a wireless access system and, more particularly, to verifying availability of an idle-mode mobile subscriber station in a wireless access system.
2. Discussion of the Background
Broadband wireless access systems typically support an idle-mode to minimize the power consumption of a mobile subscriber station (MSS). During the idle-mode, the MSS does not perform a handover procedure when moving between base stations in the same paging zone. Therefore, the MSS does not need to transmit uplink information for the handover procedure, which reduces corresponding power consumption.
A paging zone is defined as a zone that is controlled by a plurality of base stations called a paging group. Each of the base stations in the paging group has the same paging cycle (Paging_Cycle) and the same paging offset (Paging_Offset).
The MSS may make a request to a base station to switch to the idle-mode. The base station then provides a paging message including its paging-group ID, a corresponding paging cycle, and a corresponding paging offset. The corresponding MSS may thus be switched to the idle-mode. During the idle-mode, the MSS may determine whether to continue or terminate the idle-mode based on the paging message provided by the base station at each paging cycle.
If traffic needs to be transmitted by the MSS while in the idle-mode, the MSS may terminate the idle-mode. For example, the MSS may be enabled to terminate the idle-mode by the BS. Furthermore, if the MSS moves to another paging zone or loses its sync while in an idle-mode and is unable to receive a paging at a predefined time, the MSS may terminate the idle-mode.
When the MSS is in the idle-mode, the MSS normally receives a periodic paging to secure its free move in the same paging zone without performing the handover procedure as long as there exists no traffic to receive or transmit.
A procedure of an MSS entering an idle-mode is explained in detail as follows. The MSS delivers an idle-mode request message to a serving BS to enter the idle-mode. The serving BS provides a paging group ID, a paging cycle, a paging offset to the MSS via an idle-mode response message. The serving BS also releases connection information to the MSS, as well as radio resources allocated to the MSS. Furthermore, the serving BS provides a MAC address of the MSS entering the idle-mode to BSs of its paging group so that each of the BSs may page the corresponding MSS by the same paging cycle.
The MSS is allowed to enter the idle-mode via the idle-mode response message after a determination of whether there is downlink traffic that needs to be delivered to the MSS by evaluating a paging message of a broadcast format delivered from the BS according to the paging cycle. Determinations are also made of whether to perform a ranging and whether to maintain the idle-mode.
FIG. 1 is a diagram showing a format of a paging message received by an idle-mode mobile subscriber station (MSS) from a base station (BS).
Referring to FIG. 1, if the paging message states that there is downlink traffic for the MSS, the MSS terminates the idle-mode and re-registers at a network to receive the downlink traffic. If the MSS is requested to perform ranging via the paging message, the MSS performs the ranging to allow the BS to update a location of the MSS and an idle-mode MSS list. If the paging message instructs the MSS to take no action, the MSS maintains the idle-mode.
Thereafter, if uplink or downlink traffic needs to be communicated or if the paging group is handed over to another BS to terminate the MSS idle-mode, the BS informs the other BSs belonging to the same paging group so that the other BSs may remove the corresponding MSS from the idle-mode MSS list.
When the MSS is in the idle-mode, the MSS normally does not have uplink traffic. Thus, in order to provide continuous availability of the MSS, the BS delivers the paging message to the MSS to enable the corresponding MSS to perform the ranging. The BS may verify the availability of the MSS only after receiving uplink information via a ranging request from the MSS.
However, when the MSS is powered off, or when the MSS lies in a poor channel environment, the MSS may fail to receive the paging message even if the BS sets up the paging message (e.g., an action code is set to ‘0x01 (perform ranging)’ or ‘0x02 (enter network)’) for delivery. Also, in such case, the BS may not receive the ranging request message from the MSS. Thus, if the MSS is unable to normally operate, the BS may determine that the MSS is no longer available. Furthermore, the BS may need to release all management resources (e.g., MAC address for paging, etc.) for the corresponding MSS and have other BSs of the same paging group remove the corresponding MSS from their idle-mode MSS lists.